1. Field of the Invention
The present invention relates to a charge-discharge control technology of a secondary battery using lithium ion or lithium polymer, or a battery pack including the secondary battery, which is used in a portable device such as a cellular phone, a laptop computer or a portable videotape recorder with a built-in camera.
More particularly, the present invention relates to a secondary-battery control circuit, a battery pack including the secondary-battery control circuit and a portable device using the battery pack, wherein the secondary-battery control circuit is suitable to a battery management system having a remaining-charge indicating IC (Integrated Circuit) used for indicating a remaining charge, or a resetting IC used for resetting the battery management system when a power source has a predetermined voltage, the IC being a typical SBM (Smart Battery Management).
2. Description of the Related Art
A lithium-ion or lithium-polymer secondary battery is widely used in various areas of technologies such as portable devices including a cellular phone, a laptop computer, and a videotape recorder with a built-in camera. The secondary battery is often used as a battery system, by being combined with a remaining-charge indicating IC that indicates a remaining charge of the secondary battery, or a resetting IC that monitors a source voltage of the battery system and outputs a reset signal resetting the battery system when the source voltage becomes a predetermined voltage.
FIG. 1 is a circuit diagram showing a related art of the battery system described above. The related-art battery system shown in FIG. 1 includes a secondary battery 201, a protection circuit 202, a system terminal 203, cutoff switches 204 and 204′. The protection circuit 202 controls charge and discharge of the secondary battery, and protects the secondary battery from a damage caused by overcharge. The system terminal 203 is a terminal connected to a system that includes load circuits such as a re-charger, the remaining-charge indicating IC, and the resetting IC. The cutoff switches 204 and 204′ are FETs (Field Effect Transistors) that cut off a charging or discharging path between the secondary battery and the above-described system including the re-charger and various load circuits, when the secondary battery is over-charged or over-discharged.
The protection circuit 202 (the secondary-battery control circuit 2) has a function to cancel a charging operation carried out by the re-charger, by turning off the cutoff switch 204 (4), in a case of detecting an abnormal condition such as overcharge of the secondary battery, over-discharge of the secondary battery or an excess current while one of secondary batteries is being charged or discharged. Alternatively, the protection circuit 202 (the secondary-battery control circuit 2) has a function to cancel a power supply, for instance, a voltage and a load current, from the secondary battery to the system, in the case of detecting the abnormal condition while one of the secondary batteries is being charged or discharged.
According to the above-described functions of the protection circuit 202 (the secondary-battery control circuit 2), the related-art battery system shown in FIG. 1 prevents destruction of the secondary battery and a circuit on the system side.
On other hand, in the case in which the power supply from the secondary battery to the system is cut off, the system must select a unit among a plurality of units, and stop an operation of the unit, in accordance with a reason for the power supply being cut off. Thus, the system must use a circuit consisting of external components for providing the power supply to the plurality of units selectively.
The above-described related art has problems described below.
The remaining-charge indicating IC and the resetting IC among various units provided in the battery system must operate until a remaining voltage of the secondary battery becomes less than the lowest operational voltages of units other than the remaining-charge indicating IC and the resetting IC, because of their functions.
However, in a related-art technology, the power supply from the secondary battery to all the circuits including the remaining-charge indicating IC and the resetting IC is cut off, when the overcharge, the over-discharge or the excess current is detected, even in a case in which the remaining voltage of the secondary battery is higher than the lowest operational voltages of the units other than the remaining-charge indicating IC and the resetting IC. Consequently, the remaining-charge indicating IC and the resetting IC cannot function appropriately.
To solve the above-described problem, the related-art battery system shown in FIG. 1 takes steps to supply the power from the secondary battery to the remaining-charge indicating IC and the resetting IC selectively by use of the external components. Recently, portable devices using the secondary battery have become small and thin, as well as their prices have reduced. Usage of the related-art battery system using the external components cannot avoid increases in a total mounting area of the battery system and a production cost.
According to the related-art battery system as described above, the power supply from the secondary battery to the system is cut off when the overcharge, the over-discharge, or the excess current is detected. The battery system using the lithium-ion or lithium-polymer secondary battery is demanded to control monitoring of a voltage condition of the secondary battery in detail, for instance, an overcharging or over-discharging condition, because of characteristics of the battery pack. On the other hand, the battery system is demanded to continue operations performed by the remaining-charge indicating IC or the resetting IC while monitoring a current condition such as the excess current.
The related-art battery system utilizes a method of using the external components, in order to satisfy the above-described demands from the system side. However, according to such a method, the number of the external components increases, and reliability of the system decreases. Additionally, accuracy of setting values related to a power-source cutoff setting for the remaining-charge indicating IC and the resetting IC is hardly improved, and flexibility of setting the values is low, in the case of providing the external components for satisfying the above-described demands, compared to a case of including such components satisfying the above-described demands within the system.